Not really sure what makes sense to do so close to race day coming from flatland, definitely don't go too long. If you're looking for trails that are relatively flat but at 8000-9000 feet, I'd recommend one of these (most other trails around here are steep):
http://www.cospringstrails.com/hikes/centennial.html (about 8500')
http://www.mapmyhike.com/us/woodland-park-co/midland-loop-trail-course-1372313 (about 9000')
Garden of the Gods is definitely a convenient and pretty option, at a little over 6000'.
If you want to really test how you'll respond at altitude, you can drive up to the top of Pikes Peak and then hike a short way down Barr Trail and then back up. I wouldn't recommend going any farther down than the "1 mile to go" sign.
Hope that helps a little.